The telecommunications system provided in the United States and throughout the world was initially designed to transmit voice signals over high capacity transmission links, such as optical fiber, coaxial cable, and microwave. Part of the evolution of these telecommunications networks to digital technology has been the adoption of synchronous TDM transmission structures. This has led to the development of a hierarchy of TDM structures of varying capacities. The basis of the TDM hierarchy in the United States is the DS1 transmission format which multiplexes 24 channels. Each frame contains 8 bits per channel plus a framing bit or 193 bits. For voice transmission each channel contains one word of digitized voice data and the original analog voice signal is digitized using pulse code modulation (PCM) at a rate of 8,000 samples per second. Thus each channel slot and hence each frame repeats 8,000 times per second to produce a data rate of 1.544 Mbps. The same DS1 format is used to provide digital data service at the same 1.544 Mbps. data rate. The DS1 format can also be used to carry a mixture of voice and data channels.
The basic digital transmission facility is a T-1 line, which consists of an office repeater at each end feeding twisted pair wire, with digital regenerators or repeaters spaced every 6,000 feet. The function of the office repeater is to match the output of the office terminal equipment to the impedance of the line and to feed power over the line to the repeaters. The line repeaters regenerate the incoming pulses to eliminate distortion caused by the cable.
T-1 signals are kept in synchronization by loop timing in which synchronizing pulses are extracted from the incoming bit stream. The PCM output of a channel bank is encoded in bipolar format and the transition of each "1" bit is detected by the repeaters and the receiving terminals and used to keep the system in synchronization. T-1 carrier lines can be extended for about 200 miles, although most private and common carrier applications are considerably shorter because longer circuits are usually deployed over radio or fiber optic facilities.
Companies who either provide DS1 (1.544 Mbps) services or use them extensively require an economical, comprehensive method to train employees who perform DS1 testing. Telephone companies generally perform DS1 training either in a classroom environment or in the field. In the classroom, much of the training is theoretical in nature. In many cases, expensive lab equipment is available but provides only a limited range of DS1 testing scenarios. In the field, testing can be taught using actual DS1 circuits but there is limited control over test conditions.